A variety of industries (i.e., food, chemical, detergent, textile) employ microbial amylolytic enzymes to convert starch into different sugar solutions. High value is placed on the thermostability and thermoactivity of enzymes for use in the bioprocessing of starch into maltose, glucose, fructose and various sugar syrups.
Glucoamylase (EC 3.2.1.3) is an exo-acting carbohydrate which liberates glucose by hydrolysing .alpha.-1,4-linkages consecutively from the non-reducing ends of starch-type substrates. The primary application of glucoamylase is in the production of glucose syrups which can be used for either fermentation, production of crystalline glucose, or as a starting material for production of fructose syrups. The enzyme is produced by bacteria, fungi and yeasts, but fungi are used for commercial enzyme production.
Pullulanase (EC 3.2.1.41) is a debranching enzyme which attacks specifically the .alpha.-1,6 glucosidic linkages of starch and pullulan and it is commercially produced using bacteria. This enzyme is generally used in combination with the saccharifying .alpha.-amylase or glucoamylase for the production of glucose or conversion syrups because it improves the saccharification rates and yields, and also, in combination with the .beta.-amylases or maltogenic fungal .alpha.-amylases for the yield improvement in maltose production.
Commercial starch saccharification processes are usually operated at about 60.degree. C. to promote substrate solubility and to prevent interference by the growth of microorganisms. Both the known glucoamylases and the known pullulanases, however, are unstable at temperatures above 60.degree. C. Thus, the practical utility of known pullulanases or glucoamylases are somewhat restricted due to their thermal instability. Therefore, the discovery of an inexpensive source of active, thermostable glucoamylase and/or pullulanase would be an important contribution to the starch processing industry.
To date, essentially, nothing is known about the biochemical attributes of thermophilic bacteria that actively ferment starch to ethanol at temperatures of about 60.degree. C. or higher. From our earlier work we do know, however, that thermoanaerobic bacteria can often process at faster metabolic rates and produce more thermostable enzymes than mesophilic microorganisms.